1. Field of the Invention
This invention relates in general to oil well drilling equipment, and, in particular, to a method for simultaneously installing multiple strings within a wellbore, and related tools.
2. Description of Related Art
The need to install multiple strings of tubing or casing (strings) into a well is well known in the art. For example, oil and gas wells may produce at two or more different zones within the well. A separate string is often installed into the well for each zone to be produced. Additionally, strings allow an operator to control the well's production by placing special tools and devices in or on the string. An example is the use of downhole chokes or surface adjustable regulators to control flow. Additionally, multiple strings may be utilized in injection wells to inject various fluids into the adjacent formation at different depths. In such uses, each string carries fluid at a unique pressure corresponding to the depth at which the fluid is being injected into the adjacent formation.
However, the installation of multiple strings is extremely time consuming and requires specialized handling equipment. Tubing or casing strings are comprised of individual joints which typically are available in three lengths. Range 1 tubing joints are about 20 to 24 feet whereas range 2 tubing joints are about 28 to 32 feet. Range 3 casing joints are about 38 to 42 feet long. Typically, joints are connected using threaded connections. Thus, installing a string to a modest depth of 3000 feet would require from 75 to 150 individual joints depending upon the range of the tubing/casing joint used. Using conventional installation procedures, the time required to run multiple strings to this depth linearly increases with the number of strings. That is, installing two strings generally takes about one and half to twice as long as installing a single string.
Strings are installed within a wellbore by connecting successive tubing/casing joints together and lowering the resulting string into the wellbore one joint at a time. An elevator is used to lift the tubing joint at one open end, direct the tubing joint over the open end of a string protruding from the wellbore, and lower the joint to the string where the joint can be rigidly connected to the string using a threaded connection. The string is suspended within the wellbore using a slip or spider which are well known in the art. This process is repeated until the string can be lowered to the desired depth within the wellbore. If a second string is required to be installed within the wellbore, the process is repeated again with the second string, if single string elevators and slips are being used. Alternatively, dual or triple elevators and slips can be used to install up to three strings simultaneously. However, if more than three strings are required to be installed within the wellbore, then the entire process must be repeated again. Consequently, there is a need for a method and apparatus for simultaneously installing more than three strings within a wellbore.
As noted above, existing methods of simultaneously installing multiple strings in a wellbore have been limited to installing a maximum of three strings. However, such methods require a uniquely modified elevator to pick-up up to three joints as well as a specially designed slip to hold multiple strings while additional joints are being attached. Further, annular blowout preventors used for well control during the installation of strings also must be specifically designed for multiple string installations. Thus, there is a need for a method and apparatus for simultaneously installing multiple strings in a wellbore utilizing conventional single string elevators, slips, and annular blowout preventors.
Additionally, one drawback of existing methods of simultaneously installing up to three strings is that such methods cannot utilize spacers or clamps to bundle the strings in order to optimize both the spacing and orientation of the strings. Tubular joints typically are not of the same length which results in considerable variation in the length and depth of the side strings after a number of joints have been installed. When clamps or spacers are used to rigidly bundle the strings together as they are installed simultaneously within the wellbore, the side strings will extend above the wellbore to different heights. Moreover, a dual or triple elevator lifting two or three tubing joints to maneuver them over the existing strings prior to connection must necessarily lift all strings to the same height. After the connections have been made, the design of the elevators necessitates that the tops of all strings are at the same height to be lifted off the slips. Consequently, all strings cannot continue to be installed simultaneously using a multi-string elevator unless additional effort is taken to use joints of exactly the same length, or unless the side strings are individually installed within the wellbore. Either way, the installation method loses its economy when clamps and spacers are used. Thus, a need exists for a method or apparatus of simultaneously installing multiple strings within a wellbore which can be used in combination with clamps or spacers intended to optimize string spacing and orientation.
Another problem with conventional methods of installing multiple strings in a wellbore is the tendency of the strings to twist relative to each other. When strings have been installed within a wellbore and cemented in place, the strings must be perforated so the strings can communicate directly with the adjacent formation. As discussed above, each string may be directed to a specific producing or injection zone within the wellbore, thereby requiring perforation at a depth different from the adjacent strings. However, damage to adjacent strings often occurs during perforation of a string because the string twist causes the perforating charges to be inadvertently mis-directed toward adjacent strings rather than the adjacent formation.
In practice, risk of damage to adjacent strings can be mitigated, but not entirely eliminated, by using shorter perforating guns. That is, perforating guns having a shorter length can be better oriented since the strings do not twist relative to each other significantly over a short length. The tradeoff is that instead of using a single perforating gun a 100 feet long to perforate a string over a 100 foot region, ten perforating guns 10 feet long must be used to perforate a string over the same 100 feet region. Thus, there is a need for a method and apparatus of installing multiple strings in a wellbore that maintains the relative orientation of the strings (i.e., no twisting) to avoid damage to adjacent strings during perforation.
Another problem with conventional methods of installing multiple strings in a wellbore is the tendency of the strings to come into contact with each other owing to the random deviation of the well from vertical. Sufficient space between the strings is required in order to obtain cement fillage and the resulting hydraulic isolation between strings. Hydraulic isolation between strings is necessary to avoid fluid or pressure communication from one string to another. Further, a minimum distance between the strings is required where the strings are used to inject steam into the formation at different temperatures. This distance is optimized once the thermal conductivity of the cement between strings is known. If the distance is not optimal, one string will transfer heat to other strings resulting in the injection of steam into the formation at non-optimal quality (i.e., vapor content). Consequently, there is a need for a method of simultaneously installing multiple strings in a wellbore to properly space the strings from one another.
Another problem with existing methods of simultaneously installing multiple strings within a wellbore is well control during the installation of the strings. Oil well drill operators must have a means of sealing the wellbore in the event that high pressure within the wellbore forces gas or oil up through the well during installation. Typically, such means are known as blowout preventors. Annular blowout preventors are mounted below the rig floor and seal around a single string to close the wellbore and prevent the high pressure gas or oil from blowing out of the wellbore. During the simultaneous installation of multiple strings using existing methods, more than one string extends from the wellbore. Thus, annular blowout preventors must be modified to seal around more than a single string in such installations. The modification is specific to the number of strings being installed. Thus, there is a need for an apparatus and method of installing multiple strings within a wellbore which can be used with less costly and readily available single string annular blowout preventors for well control.
Accordingly, there has existed a definite need for a simple and economical method and apparatus for simultaneously installing multiple strings within a wellbore in a manner which addresses the shortcomings present in existing methods as identified above. The present invention satisfies these needs and provides further related advantages.